1. Field of the Invention
The present invention relates to a process for producing a substrate which comprises firing a green sheet comprising a powder of borosilicate glass.
2. Discussion of Background
Heretofore, it has been common to produce a circuit substrate by applying a slurry containing a powder of borosilicate glass onto a flat plate, followed by drying to obtain a green sheet, then printing a conductive pattern of a conductive ink on the surface of the green sheet, followed by firing. Such a green sheet comprising a powder of borosilicate glass can be sintered at a temperature of not higher than 1,000° C., and accordingly, it can be fired simultaneously with a low resistance conductor such as Cu, Ag or Au, and is thus industrially advantageous. Further, B2O3 in the borosilicate glass has a function to suppress heat shrinkage of glass, and is preferred also with a view to improving dimensional precision or suppressing a strain with the conductive pattern. Further, as the boron concentration is high, it is possible to suppress the dielectric constant and the dielectric loss to be low, thus leading to a merit such that it is possible to obtain a substrate excellent in the electrical characteristics.
However, with a green sheet employing a powder of borosilicate glass, it may happen that moisture in air will act on boron in the surface layer to precipitate crystals of boric acid or boron hydroxide (bleeding of boric acid), whereby printing irregularity is likely to result at the time of forming a conductive pattern, thus leading to disconnection. In order to prevent such bleeding of boric acid, it has been proposed to add an alumina powder to a powder of borosilicate glass and carry out preliminary firing at a temperature of at least the glass transition point and at most the softening point of the borosilicate glass thereby to have B2O3 and alumina reacted (Patent Document 1).
Patent Document 1: JP-A-10-120439
However, according to Patent Document 1, powder particles tend to be bonded one another to form relatively large agglomerates, which are required to be again pulverized before making a slurry, whereby the number of steps is likely to be increased, thus leading to an increase of the cost. Further, a non-reacted alumina powder may remain and make it difficult to control the composition of the desired green sheet.